A distributed denial of service attack (“DDoS”) occurs when multiple compromised systems flood the bandwidth or resources of a targeted system, usually a web server. For example, one type of DDoS mechanism is triggered on a specific date and time. At that specific date and time, thousands of compromised machines on the internet will access a target web server at the same time, which brings the web server down due to resource exhaustion. There are several defense mechanisms being used or researched today. These mechanisms include ingress filtering to stop the attack at the target, trace back to catch and stop the attacker at the source, and resource management and congestion control. All these defense techniques face increasing challenges as attack detection and trace back become more and more difficult.